As well known by people involved in the art, chromatographic systems rely on the use of valves to allow reproducible sample introduction and various column switching schemes.
For the last forty years, many people have designed diaphragm valves for chromatography. Such diaphragm valves have been used in many commercially available gas chromatographs. They are apt to be integrated more easily in a gas chromatograph due to their physical size and since the actuator is embedded in the valve itself. These characteristics make them attractive for gas chromatograph manufacturers.
Referring to FIG. 1 (PRIOR ART), there is shown an example of a typical diaphragm-sealed valve as known in the art. The valve 1 is provided with a top block 2 having an interface 4 and a plurality of ports 6. Each of the ports 6 opens at the interface 4 and has an inclined thread passage 8 to connect various analytical fitting and tubing (not shown). At the bottom of the inclined thread passage 8, there is a conduit 10 extending in the top block 2 and opening at the interface 4. The ports 6 are arranged on a circular line on the interface 4 of the top block 2. The interface 4 is advantageously flat and polished to minimize leaks between ports and from the ambient atmosphere. The valve 1 is also provided with a bottom block 12 and a diaphragm 14, which is generally made of polyimide, Teflon or other polymer material. The diaphragm 14 is positioned between the top block interface 4 and the bottom block 12, and has a recess therein extending along the circular line formed by the ports 6 and biased away from the interface 4 of the top block 2. The recess 18 in the diaphragm 14 sits in a matching recess 20 made in the bottom block 12, thereby allowing some clearance for fluid circulation between adjacent ports 6.
The valve 1 is also provided with a plurality of plungers 16 mounted in the bottom block 12, each being respectively arranged to be able to compress the diaphragm 14 against the top block 2 at a position located between two of the ports 6. Preferably, as illustrated, when the valve is at rest, three plungers 16 are up while the other three are down. When the plungers are up, they compress the diaphragm 14 against the top block 2 and close the conduits made by the diaphragm recess 18, so that fluid circulation is blocked. The bottom block 12 keeps the plungers 16 and the actuating mechanism in position.
The performance of valves of the type shown in FIG. 1 is generally poor. The leak rate from port to port is too high for most applications and thus limits the system performance. Moreover, the pressure drop on the valve's ports differs from port to port, causing pressure and flow variations in the system. This causes detrimental effects on column performance and detector baseline. Furthermore, many valve designs allow for unacceptable inboard contamination.
Known prior art includes many variations of the valve shown in FIG. 1 (PRIOR ART) attempting to prevent or minimize inboard and outboard contamination and generally improve the performance of diaphragm valves.
However, one drawback still not resolve is the contamination of process gas by ambient air or light gas located between the under surface of the diaphragm 14 and the upper surface of the bottom block 12. The gas located underneath the diaphragm 14 mainly comes from the activation of the plungers 16, and may also be due to inboard leaks. The gas located underneath the diaphragm 14 permeates upwardly through it into the recess 18 and thereby contaminates the process gas circulating in the recess 18. This problem more frequently occurs in application in which light gas is used, such as helium or hydrogen. In other situations, hazardous process gas, such as silane, may leak or diffuse through the diaphragm and damage the valve.
Another drawback of existing design is the difficulty to operate such valves when used in sub-atmospheric applications, or in high pressure applications, for example when pressures in the order of thousands of pounds per squared inch are applied. When such high pressures are used, the diaphragm is pressed downwardly into the matching recess 20 in the bottom block 12, and it becomes difficult for the plungers 16 to compress the diaphragm upwardly against the interface 4 of the top block 2.
There is therefore a need for an improved diaphragm-sealed valve.